This invention relates to a process for producing micro lens.
A micro lens, in general, refers to a lens having a small diameter (for example, about 1 mm or less, though not very strict). Such a micro lens is conventionally produced by several methods as set forth below.
In the first method, an ultraviolet ray is irradiated to a flat plate of a photosensitive glass material through a matrix member obtained by arranging a plurality of circular shielding portions in a matrix pattern. That portion of the flat plate, to which the ultraviolet ray is irradiated, is crystallized and contracted in volume in the next following heating process. As a consequence, the remaining portion of the flat plate, to which the ultraviolet ray is not irradiated, is expanded under the effect of a compressive force and allowed to project from a surface of the glass plate to thereby form a convexly curved surface. Each portion having the convexly curved surface becomes the micro lens. In this way, a plurality of micro lenses are formed on the glass plate in a two-dimensional matrix pattern to provide a micro lens array.
The first method has such a drawback that only special glass material having photosensitive properties can be used. Further, since a heating process for crystallizing the material is required after the irradiation of the ultraviolet ray, the manufacturing cost is high.
In the second method, a mask having a number of circular opening portions formed in a matrix pattern is coated on a flat plate made of a glass material. The glass plate is then dipped into a solution containing ions for providing a high refractive index. As a result, ion exchange is taken place only at the surface of the glass plate corresponding to the opening portions of the mask and the refractive index is locally distributed. Each portion of the glass plate having such a distribution of refractive index becomes a micro lens. In this way, a number of such micro lenses are formed on the glass plate in a matrix pattern to provide a micro lens array.
The second method has such drawbacks that the ion exchange process is troublesome because the process is executed at a high temperature for a long time and the manufacturing cost is high.
The third method is disclosed in a thesis under the heading of "processing of Glass Micro Lens Array by Co.sub.2 Laser" issued February, 1987 by "The Review of Laser Engineering". According to this method, a soda glass plate having a comparatively low glass transition point is heated at 300.degree. C. Parallel laser beam are converged by a lens and irradiated to the surface of the heated glass plate. By doing this, the refractive index is locally varied to form a micro lens. It should be noted here that the surface of the glass plate is flat as in the case with the second method and only the refractive index is locally varied.
The third method has such drawbacks that the manufacturing cost is high because heating is required, and it is difficult to symmetrically distribute the refractive index. Moreover, since the gradient of the refractive index spreads over a comparative wide range, it is impossible to form a number of micro lenses at narrow spaces.